This invention relates to improvements in circuits for driving focus actuators of the "double solenoid" variety. Such actuators are useful, for example, in the fields of optical recording and playback for maintaining a read/write laser beam in sharp focus on a moving optical recording element.
In the commonly assigned U.S. application Ser. No. 263,441 filed on Oct. 27, 1988 in the name of J. S. Chandler and entitled "Double Solenoid Focus Actuator For Optical Recording/Playback Apparatus," there is disclosed a focus actuator for controlling the focal position of a lens used in an optical recording system to focus a laser beam on a recording element. Such actuator comprises a pair of juxtaposed coils which concentrically surround a ferromagnetic lens barrel which supports the focusing lens. When current is applied to one coil or the other, the lens barrel moves along the lens axis in an attempt to minimize the magnetic path between the energized coil and the lens barrel. In effect, the actuator behaves as a pair of solenoids mounted back-to-back with the lens barrel serving as a shared core piece or plunger. An advantage of this particular type of focus actuator over similar devices is that it requires no permanent magnet.
In the above-noted application, a simple circuit is disclosed for energizing one or the other of the actuator coils. Such circuit comprises a diode arrangement which, when connected to a signal of alternating polarity, allows current to flow in either one of the coils, depending on the signal polarity. While this driving circuit operates well with input signals of relatively large amplitude, it cannot respond to input signals of relatively low amplitude, as is required to maintain a "fine" focus condition. The problem with such diode circuits is that the turn on voltage of each diode is typically about 0.7 volts. Thus, since the circuit will not respond to voltages of lesser value, there is a dead-band discontinuity in the system until the control signal exceeds about 0.7 volts. Ideally, the focus actuator should respond to the smallest of signals in order to position the focusing lens with the precision required for high density optical recording.
In addition to the dead-band discontinuity problem mentioned above, another characteristic of the double solenoid focus actuator is that its response is non-linear; in fact, its response is substantially quadratic, the lens displacement varying with the square of the applied coil current. Such a response can render the focus servo system unstable for low-amplitude focus servo signals.